1. Field of the Invention
The present invention relates to a fixing member which is used in the field of fixing technology for electrophotographic image forming apparatus such as a photocopier as well as LBP, and which has a rubber based mold-releasing layer and capable of fixing a toner onto a recording material especially in an oilless system, a fixing apparatus and a fixing method.
2. Related Background Art
A fixing member is incorporated into an electrophotographic image forming apparatus in order to heat and fix a toner image formed on a recording material (hereinafter also to be referred to as a transfer material) such as paper. Such fixing member is required to have a toner mold-releasing property. A toner mold-releasing property means to what extent a toner will not be attached to the surface of a fixing member when to bring a toner image formed electrostatically onto the recording material into fixing with heat and pressure. When a toner offset, that is, attachment of toner onto the surface of fixing member, takes place, partial lacking in the toner image formed on the recording material takes place or offset toner is refixed onto the subsequent recording material to occasionally give rise to a problem to worsen the image. Accordingly, use of a material excellent in toner releasing property on the surface of the fixing member, that is, a mold-releasing layer, is important as properties of the member.
As such a fixing member, the one with at least one mold-releasing layer being formed on the outer periphery of the cylindrical axis body and the mold-releasing layer of which is formed with a fluorocarbon resin is generally being used. The surface energy of a fluorocarbon resin is low and non-adhesive and excellent in a mold-releasing property. However, since it is a resin, its hardness is intensive, and at the time when the toner image formed electrostatically is brought into fixing with heat and pressure, the toner particles more than necessary in number are crushed, resulting in fine lines extending and overlapping, making high quality image hardly obtainable such as by occasionally making small letters difficult to be read and the like.
In contrast thereto, those whose mold-releasing layer is formed with rubber being an elastic body is more flexible compared with resins and do not crush toner particles more than necessary, and is advantageous in readily giving rise to a quality image. However, in general a rubber provides higher surface energy compared with a fluorocarbon resin and occasionally is inferior in a toner mold-releasing property. In particular, as for color oilless fixing, it is only fluorocarbon resins that are available for practical use as a material for a mold-releasing layer but no fixing members comprising a mold-releasing layer containing rubber (hereinafter also to be referred to as “rubber mold-releasing layer”) have been realized yet for practical use to date. In order to obtain quality images, a mold-releasing layer containing flexible rubber is being desired instead of a mold-releasing layer containing hard resin.
Since a fixing member is used under a high temperature around 200° C., a rubber for a mold-releasing layer is required to be heat resistant. Such a heat resistant rubber type includes fluorocarbon rubbers and silicone rubbers.
A fluorocarbon rubber is a rubber being an elastic body by adding a cross-linking agent, cross-linking auxiliary agent or reinforcing combination preparation and the like to various kinds of fluorocarbon rubber polymers (hereinafter also to be referred to as fluoropolymer) configured by comprising a partially fluorinated hydrocarbon chain to form a three-dimensional network by cross-linking such as cross-linking by a polyamine, cross-linking by a polyol or cross-linking by an organic peroxide.
So far, a fluorocarbon rubber is mainly used as a mold-releasing layer of an oil coating system for supplying a color apparatus with degenerative silicone oil from outside. As types of fluoropolymer, those fluoropolymers configured by comprising vinylidene fluoride-hexafluoropropylene copolymer, or vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer subject to combination with metal containing filler or reinforcing filler and thereafter brought into polyamine cross-linking or polyol cross-linking are mainly used.
In addition, a silicone rubber refers to a rubber being an elastic body by compounding various kinds of fillers corresponding with a necessity with a raw rubber such as a polymer configured by methylvinyl siloxane or methylphenylvinyl siloxane unit subject to substitution of a part of a methyl group with a phenyl group to form a three-dimensional network by cross-linking such as an addition reaction cross-linking or cross-linking by an organic peroxide. In general, being excellent in heat resistance, a silicone rubber is mainly used as a heat conductive elastic layer for a bottom layer of a mold-releasing layer. In addition, as a mold-releasing layer, it has been mainly used as a mold-releasing layer of an oil coating system for supplying a color apparatus with dimethyl silicone oil from outside.
Recently, in order to ensure a mold-releasing property of toner, an oilless fixing system which does not undergo oil coating from outside as described above but adds a wax for extending mold-releasing assisting effects to inside toner particles is realized for practical use in a mold-releasing layer made of fluorocarbon resin.
In order to obtain a quality image, adoption of fluorocarbon rubber instead of fluorocarbon resin imparts a low affinity between the wax in the toner and the fluorocarbon rubber, and the wax does not extend its mold-releasing assisting effects sufficiently. That occasionally has given rise to paper twining or toner offset as a result of unseparation of toner from the mold-releasing layer. In addition, use of dimethyl silicone rubber as the mold-releasing layer imparts high affinity between the wax in the toner and the dimethyl silicone rubber, but too much affinity affects the toner mold-releasing property somehow adversely and, likewise in case of the fluorocarbon rubber, occasionally has given rise to paper twining or toner offset.
As for oilless fixing for a color apparatus, the Japanese Patent Application Laid-Open No. 2000-242115 has proposed an image recording apparatus, wherein a toner comprises toner particles containing at least a wax (hereinafter also referred to as wax-containing toner) and, in a fixing apparatus, a fixing member surface on the side in contact with an unfixed toner image comprises a mold-releasing layer configured by comprising a fluorocarbon rubber with high affinity with the wax. In addition, Japanese Patent No. 3055119 has proposed a fixing apparatus, wherein a toner comprises toner particles containing at least a binder resin, a colorant and a wax, the average molecule by weight (Mw) of the wax being 400 to 4000 and its average molecule by number (Mn) being 200 to 4000, and a mold-releasing layer disposed on the outest surface of a heat fixing member is formed by mixture of fluorocarbon rubber and tetrafluoroethylene-perfluoroalkylvinylether copolymer, and the contact angle of the mold-releasing layer with water is adjusted to fall within the range of 80 to 103°.
The above described Japanese Patent Application Laid-Open No. 2000-242115 has proposed a mold-releasing layer configured by comprising a fluorocarbon rubber having a high wax affinity, but further improvement in toner mold-releasing property is desired.
In addition, the above described Japanese Patent No. 3055119 has proposed a mold-releasing layer configured by mixture of fluorocarbon rubber and tetrafluoroethylene-perfluoroalkylvinylether copolymer for the purpose of refraining from deteriorating the affinity with a wax. However, this is not a single rubber body but a mixture of rubber and resin, flexibility of which occasionally may be low compared with a single rubber body.